Use cryptographic PRNGs when available.

This means node's crypto.randomBytes on the server, and
window.crypto.getRandomValues on the client. If node's crypto.randomBytes throws
an exception, we fall back to crypto.pseudoRandomBytes. If
window.crypto.getRandomValues isn't supported by the browser, we fall back to
the alea generator that we had been using previously.
Merge branch 'csprng' into devel